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Abstract:

A lithographic apparatus, includes a support structure configured to hold
a patterning device, the patterning device configured to impart a beam of
radiation with a pattern in its cross-section; a substrate table
configured to hold a substrate; a projection system configured to project
the patterned beam onto a target portion of the substrate; a liquid
supply system configured to provide liquid to a space between the
projection system and the substrate table; a sensor configured to measure
an exposure parameter using a measuring beam projected through the
liquid; and a correction system configured to determine an offset based
on a change of a physical property impacting a measurement made using the
measuring beam to at least partly correct the measured exposure
parameter.

Claims:

1. A lithographic apparatus, comprising: a support structure configured
to hold a patterning device, the patterning device configured to impart a
beam of radiation with a pattern in its cross-section; a substrate table
configured to hold a substrate; a projection system configured to project
the patterned beam onto a target portion of the substrate; a liquid
supply system configured to provide liquid to a space between the
projection system and the substrate table; a sensor configured to measure
an exposure parameter using a measuring beam projected through the
liquid; and a correction system configured to determine an offset based
on a change of a physical property impacting a measurement made using the
measuring beam to at least partly correct the measured exposure
parameter.

2. The apparatus according to claim 1, wherein the physical property
comprises (a) a temperature, or (b) a pressure, or (c) a composition, or
(d) any combination of (a)-(c), of the liquid.

3. The apparatus according to claim 1, wherein the physical property
comprises (a) a temperature, or (b) a pressure, or (c) a composition, or
(d) any combination of (a)-(c), of a flushing gas, of or on an optical
element of the projection system, or both.

4. The apparatus according to claim 2, further comprising a temperature
sensor configured to measure the temperature of the liquid, a pressure
sensor configured to measure the pressure of the liquid, or both.

5. The apparatus according to claim 1, wherein the exposure parameter
comprises a focus of the patterned beam, a height of the substrate, or
both.

6. The apparatus according to claim 1, wherein the exposure parameter
comprises a lateral placement of the patterned beam, an X-Y position of
the substrate, or both.

7. The apparatus according to claim 1, wherein the physical property
comprises a wavelength of the measuring beam, a wavelength of the
patterned beam, or both.

8. The apparatus according to claim 1, wherein the offset comprises a
multiplication of the change of the physical property and a rate of
change of the exposure parameter with respect to the physical property.

9. The apparatus according to claim 1, wherein the sensor is configured
to project the measuring beam through an optical element of the
projection system through which the patterned beam is to be projected.

10. The apparatus according to claim 1, wherein the correction system is
configured to determine the offset based on the difference between a
change of the physical property for a wavelength of the measuring beam
and a change of the physical property for a wavelength of the patterned
beam.

11. A lithographic apparatus, comprising: a substrate table configured to
hold a substrate; a projection system configured to project a patterned
beam onto a target portion of the substrate, the projection system having
an optical element; a liquid supply system configured to provide liquid
to a space between the projection system and the substrate table, the
optical element configured to be connected to the liquid; and a sensor
configured to measure a height of the optical element.

12. The apparatus according to claim 11, wherein the optical element is
moveable and further comprising a correction system configured to move
the optical element to at least partly correct for a change in height of
the optical element.

13. The apparatus according to claim 11, further comprising a correction
system configured to move the substrate table to at least partly correct
for a change in height of the optical element.

14. The apparatus according to claim 11, wherein the sensor is configured
to detect a measuring beam traveling from the optical element to measure
the height of the optical element.

15. The apparatus according to claim 14, wherein sensor is further
configured to project the measuring beam at the optical element.

16. The apparatus according to claim 11, wherein the sensor is configured
to detect a measuring beam that reflects from the optical element and
configured to measure the height of the optical element by evaluating
fringes created by interference between the measuring beam and a beam
reflected from the substrate or substrate table.

17. The apparatus according to claim 11, wherein the sensor is a level
sensor configured to measure a height of the substrate or substrate
table.

18. The apparatus according to claim 11, wherein a change in the height
of the optical element is a measure of focus error, spherical aberration,
or both, and further comprising a correction system configured to at
least partly correct such focus error, spherical aberration, or both.

Description:

[0001] This application is a continuation application of U.S. patent
application Ser. No. 12/694,880, filed Jan. 27, 2010, now allowed, which
is a divisional application of U.S. patent application Ser. No.
11/298,942, filed Dec. 12, 2005, now U.S. Pat. No. 7,670,730, which is a
continuation-in-part application of U.S. patent application Ser. No.
11/025,603, filed Dec. 30, 2004, now abandoned, the content of which is
herein incorporated in its entirety by reference.

FIELD

[0002] The invention relates to a lithographic apparatus and a method for
manufacturing a device.

BACKGROUND

[0003] A lithographic apparatus is a machine that applies a desired
pattern onto a substrate, usually onto a target portion of the substrate.
A lithographic apparatus can be used, for example, in the manufacture of
integrated circuits (ICs). In that instance, a patterning device, which
is alternatively referred to as a mask or a reticle, may be used to
generate a circuit pattern to be formed on an individual layer of the IC.
This pattern can be transferred onto a target portion (e.g. comprising
part of, one, or several dies) on a substrate (e.g. a silicon wafer).
Transfer of the pattern is typically via imaging onto a layer of
radiation-sensitive material (resist) provided on the substrate. In
general, a single substrate will contain a network of adjacent target
portions that are successively patterned. Known lithographic apparatus
include so-called steppers, in which each target portion is irradiated by
exposing an entire pattern onto the target portion at one time, and
so-called scanners, in which each target portion is irradiated by
scanning the pattern through a radiation beam in a given direction (the
"scanning"-direction) while synchronously scanning the substrate parallel
or anti-parallel to this direction. It is also possible to transfer the
pattern from the patterning device to the substrate by imprinting the
pattern onto the substrate.

[0004] It has been proposed to immerse the substrate in the lithographic
projection apparatus in a liquid having a relatively high refractive
index, e.g. water, so as to fill a space between the final element of the
projection system and the substrate. The point of this is to enable
imaging of smaller features since the exposure radiation will have a
shorter wavelength in the liquid. (The effect of the liquid may also be
regarded as increasing the effective numerical aperture (NA) of the
system and also increasing the depth of focus.) Other immersion liquids
have been proposed, including water with solid particles (e.g. quartz)
suspended therein.

[0005] However, submersing the substrate or substrate and substrate table
in a bath of liquid (see, for example, United States patent U.S. Pat. No.
4,509,852, hereby incorporated in its entirety by reference) means that
there is a large body of liquid that must be accelerated during a
scanning exposure. This requires additional or more powerful motors and
turbulence in the liquid may lead to undesirable and unpredictable
effects.

[0006] One of the solutions proposed is for a liquid supply system to
provide liquid on only a localized area of the substrate and in between
the final element of the projection system and the substrate (the
substrate generally has a larger surface area than the final element of
the projection system). One way which has been proposed to arrange for
this is disclosed in PCT patent application no. WO 99/49504, hereby
incorporated in its entirety by reference. As illustrated in FIGS. 2 and
3, liquid is supplied by at least one inlet IN onto the substrate,
preferably along the direction of movement of the substrate relative to
the final element, and is removed by at least one outlet OUT after having
passed under the projection system. That is, as the substrate is scanned
beneath the element in a -X direction, liquid is supplied at the +X side
of the element and taken up at the -X side. FIG. 2 shows the arrangement
schematically in which liquid is supplied via inlet IN and is taken up on
the other side of the element by outlet OUT which is connected to a low
pressure source. In the illustration of FIG. 2 the liquid is supplied
along the direction of movement of the substrate relative to the final
element, though this does not need to be the case. Various orientations
and numbers of in- and out-lets positioned around the final element are
possible, one example is illustrated in FIG. 3 in which four sets of an
inlet with an outlet on either side are provided in a regular pattern
around the final element.

SUMMARY

[0007] It would be advantageous, for example, to provide a method,
apparatus and/or computer program product for correcting an exposure
parameter of an immersion lithographic apparatus.

[0008] According to an aspect of the invention, there is provided a method
for correcting an exposure parameter of an immersion lithographic
apparatus, the method comprising:

[0009] measuring an exposure parameter using a measuring beam projected
through a liquid between the projection system and a substrate table of
the immersion lithographic apparatus; and

[0010] determining an offset based on a change of a physical property
impacting a measurement made using the measuring beam to at least partly
correct the measured exposure parameter.

[0011] According to an aspect of the invention, there is provided a
lithographic apparatus, comprising:

[0012] a support structure configured to hold a patterning device, the
patterning device configured to impart a beam of radiation with a pattern
in its cross-section;

[0013] a substrate table configured to hold a substrate;

[0014] a projection system configured to project the patterned beam onto a
target portion of the substrate;

[0015] a liquid supply system configured to provide liquid to a space
between the projection system and the substrate table;

[0016] a sensor configured to measure an exposure parameter using a
measuring beam projected through the liquid; and

[0017] a correction system configured to determine an offset based on a
change of a physical property impacting a measurement made using the
measuring beam to at least partly correct the measured exposure
parameter.

[0018] According to an aspect of the invention, there is provided a
computer program product for correcting an exposure parameter of an
immersion lithographic apparatus, comprising:

[0019] software code configured to measure an exposure parameter using a
measuring beam projected through a liquid between the projection system
and a substrate table of the immersion lithographic apparatus; and

[0020] software code configured to determine an offset based on a change
of a physical property impacting a measurement made using the measuring
beam to at least partly correct the measured exposure parameter.

[0021] According to an aspect of the invention, there is provided a
lithographic apparatus, comprising:

[0022] a substrate table configured to hold a substrate;

[0023] a projection system configured to project a patterned beam onto a
target portion of the substrate, the projection system having an optical
element;

[0024] a liquid supply system configured to provide liquid to a space
between the projection system and the substrate table, the optical
element configured to be connected to the liquid; and

[0025] a sensor configured to measure a height of the optical element.

[0026] According to an aspect of the invention, there is provided a method
of correcting for an imaging error of an immersion lithographic
apparatus, comprising:

[0027] measuring a height of an optical element of a projection system in
the immersion lithographic apparatus, the optical element connected to a
liquid between the projection system and a substrate table of the
projection system; and

[0028] at least partly correcting the image error by moving the optical
element, moving the substrate table, or both.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Embodiments of the invention will now be described, by way of
example only, with reference to the accompanying schematic drawings in
which corresponding reference symbols indicate corresponding parts, and
in which:

[0030]FIG. 1 depicts a lithographic apparatus according to an embodiment
of the invention;

[0031] FIGS. 2 and 3 depict a liquid supply system for use in a
lithographic projection apparatus;

[0032]FIG. 4 depicts another liquid supply system for use in a
lithographic projection apparatus;

[0033] FIG. 5 depicts another liquid supply system for use in a
lithographic projection apparatus;

[0034]FIG. 6 schematically depicts passage of a radiation beam through an
optical element of the projection system of a lithographic apparatus
according to an embodiment of the invention;

[0035]FIG. 7 depicts a flow chart of a method according to an embodiment
of the invention;

[0036]FIG. 8 schematically depicts a sensor used to measure a height or
change of height of an optical element of the projection system of a
lithographic apparatus according to an embodiment of the invention; and

[0037]FIG. 9 schematically depicts a sensor used to measure a height or
change of height of an optical element of the projection system of a
lithographic apparatus according to an embodiment of the invention.

DETAILED DESCRIPTION

[0038]FIG. 1 schematically depicts a lithographic apparatus according to
one embodiment of the invention. The apparatus comprises: [0039] an
illumination system (illuminator) IL configured to condition a radiation
beam PB (e.g. UV radiation or DUV radiation); [0040] a support structure
(e.g. a mask table) MT constructed to support a patterning device (e.g. a
mask) MA and connected to a first positioner PM configured to accurately
position the patterning device in accordance with certain parameters;
[0041] a substrate table (e.g. a wafer table) WT constructed to hold a
substrate (e.g. a resist-coated wafer) W and connected to a second
positioner PW configured to accurately position the substrate in
accordance with certain parameters; and [0042] a projection system (e.g.
a refractive projection lens system) PL configured to project a pattern
imparted to the radiation beam PB by patterning device MA onto a target
portion C (e.g. comprising one or more dies) of the substrate W.

[0043] The illumination system may include various types of optical
components, such as refractive, reflective, magnetic, electromagnetic,
electrostatic or other types of optical components, or any combination
thereof, for directing, shaping, or controlling radiation.

[0044] The support structure supports, i.e. bears the weight of, the
patterning device. It holds the patterning device in a manner that
depends on the orientation of the patterning device, the design of the
lithographic apparatus, and other conditions, such as for example whether
or not the patterning device is held in a vacuum environment. The support
structure can use mechanical, vacuum, electrostatic or other clamping
techniques to hold the patterning device. The support structure may be a
frame or a table, for example, which may be fixed or movable as required.
The support structure may ensure that the patterning device is at a
desired position, for example with respect to the projection system. Any
use of the terms "reticle" or "mask" herein may be considered synonymous
with the more general term "patterning device."

[0045] The term "patterning device" used herein should be broadly
interpreted as referring to any device that can be used to impart a
radiation beam with a pattern in its cross-section such as to create a
pattern in a target portion of the substrate. It should be noted that the
pattern imparted to the radiation beam may not exactly correspond to the
desired pattern in the target portion of the substrate, for example if
the pattern includes phase-shifting features or so called assist
features. Generally, the pattern imparted to the radiation beam will
correspond to a particular functional layer in a device being created in
the target portion, such as an integrated circuit.

[0046] The patterning device may be transmissive or reflective. Examples
of patterning devices include masks, progammable mirror arrays, and
programmable LCD panels. Masks are well known in lithography, and include
mask types such as binary, alternating phase-shift, and attenuated
phase-shift, as well as various hybrid mask types. An example of a
programmable mirror array employs a matrix arrangement of small mirrors,
each of which can be individually tilted so as to reflect an incoming
radiation beam in different directions. The tilted mirrors impart a
pattern in a radiation beam which is reflected by the mirror matrix.

[0047] The term "projection system" used herein should be broadly
interpreted as encompassing any type of projection system, including
refractive, reflective, catadioptric, magnetic, electromagnetic and
electrostatic optical systems, or any combination thereof, as appropriate
for the exposure radiation being used, or for other factors such as the
use of an immersion liquid or the use of a vacuum. Any use of the term
"projection lens" herein may be considered as synonymous with the more
general term "projection system".

[0048] As here depicted, the apparatus is of a transmissive type (e.g.
employing a transmissive mask). Alternatively, the apparatus may be of a
reflective type (e.g. employing a programmable mirror array of a type as
referred to above, or employing a reflective mask).

[0049] The lithographic apparatus may be of a type having two (dual stage)
or more substrate tables (and/or two or more mask tables). In such
"multiple stage" machines, the additional tables may be used in parallel,
or preparatory steps may be carried out on one or more tables while one
or more other tables are being used for exposure.

[0050] Referring to FIG. 1, the illuminator IL receives a radiation beam
from a radiation source SO. The source and the lithographic apparatus may
be separate entities, for example when the source is an excimer laser. In
such cases, the source is not considered to form part of the lithographic
apparatus and the radiation beam is passed from the source SO to the
illuminator IL with the aid of a beam delivery system BD comprising, for
example, suitable directing mirrors and/or a beam expander. In other
cases the source may be an integral part of the lithographic apparatus,
for example when the source is a mercury lamp. The source SO and the
illuminator IL, together with the beam delivery system BD if required,
may be referred to as a radiation system.

[0051] The illuminator IL may comprise an adjuster AD for adjusting the
angular intensity distribution of the radiation beam. Generally, at least
the outer and/or inner radial extent (commonly referred to as
σ-outer and σ-inner, respectively) of the intensity
distribution in a pupil plane of the illuminator can be adjusted. In
addition, the illuminator IL may comprise various other components, such
as an integrator IN and a condenser CO. The illuminator may be used to
condition the radiation beam, to have a desired uniformity and intensity
distribution in its cross-section.

[0052] The radiation beam PB is incident on the patterning device (e.g.,
mask MA), which is held on the support structure (e.g., mask table MT),
and is patterned by the patterning device. Having traversed the mask MA,
the radiation beam PB passes through the projection system PL, which
focuses the beam onto a target portion C of the substrate W. An immersion
hood IH, which is described further below, supplies immersion liquid to a
space between the final element of the projection system PL and the
substrate W.

[0053] With the aid of the second positioner PW and position sensor IF
(e.g. an interferometric device, linear encoder or capacitive sensor),
the substrate table WT can be moved accurately, e.g. so as to position
different target portions C in the path of the radiation beam PB.
Similarly, the first positioner PM and another position sensor (which is
not explicitly depicted in FIG. 1) can be used to accurately position the
mask MA with respect to the path of the radiation beam PB, e.g. after
mechanical retrieval from a mask library, or during a scan. In general,
movement of the mask table MT may be realized with the aid of a
long-stroke module (coarse positioning) and a short-stroke module (fine
positioning), which form part of the first positioner PM. Similarly,
movement of the substrate table WT may be realized using a long-stroke
module and a short-stroke module, which form part of the second
positioner PW. In the case of a stepper (as opposed to a scanner) the
mask table MT may be connected to a short-stroke actuator only, or may be
fixed. Mask MA and substrate W may be aligned using mask alignment marks
Ml, M2 and substrate alignment marks P1, P2. Although the substrate
alignment marks as illustrated occupy dedicated target portions, they may
be located in spaces between target portions (these are known as
scribe-lane alignment marks). Similarly, in situations in which more than
one die is provided on the mask MA, the mask alignment marks may be
located between the dies.

[0054] The depicted apparatus could be used in at least one of the
following modes:

[0055] 1. In step mode, the mask table MT and the substrate table WT are
kept essentially stationary, while an entire pattern imparted to the
radiation beam is projected onto a target portion C at one time (i.e. a
single static exposure). The substrate table WT is then shifted in the X
and/or Y direction so that a different target portion C can be exposed.
In step mode, the maximum size of the exposure field limits the size of
the target portion C imaged in a single static exposure.

[0056] 2. In scan mode, the mask table MT and the substrate table WT are
scanned synchronously while a pattern imparted to the radiation beam is
projected onto a target portion C (i.e. a single dynamic exposure). The
velocity and direction of the substrate table WT relative to the mask
table MT may be determined by the (de-)magnification and image reversal
characteristics of the projection system PL. In scan mode, the maximum
size of the exposure field limits the width (in the non-scanning
direction) of the target portion in a single dynamic exposure, whereas
the length of the scanning motion determines the height (in the scanning
direction) of the target portion.

[0057] 3. In another mode, the mask table MT is kept essentially
stationary holding a programmable patterning device, and the substrate
table WT is moved or scanned while a pattern imparted to the radiation
beam is projected onto a target portion C. In this mode, generally a
pulsed radiation source is employed and the programmable patterning
device is updated as required after each movement of the substrate table
WT or in between successive radiation pulses during a scan. This mode of
operation can be readily applied to maskless lithography that utilizes
programmable patterning device, such as a programmable mirror array of a
type as referred to above.

[0058] Combinations and/or variations on the above described modes of use
or entirely different modes of use may also be employed.

[0059] A further immersion lithography solution with a localized liquid
supply system is shown in FIG. 4. Liquid is supplied by two groove inlets
IN on either side of the projection system PL and is removed by a
plurality of discrete outlets OUT arranged radially outwardly of the
inlets IN. The inlets IN and OUT can be arranged in a plate with a hole
in its center and through which the projection beam is projected. Liquid
is supplied by one groove inlet IN on one side of the projection system
PL and removed by a plurality of discrete outlets OUT on the other side
of the projection system PL, causing a flow of a thin film of liquid
between the projection system PL and the substrate W. The choice of which
combination of inlet IN and outlets OUT to use can depend on the
direction of movement of the substrate W (the other combination of inlet
IN and outlets OUT being inactive).

[0060] Another immersion lithography solution with a localized liquid
supply system solution which has been proposed is to provide the liquid
supply system with a liquid confinement structure which extends along at
least a part of a boundary of the space between the final element of the
projection system and the substrate table. Such a solution is illustrated
in FIG. 5. The liquid confinement structure is substantially stationary
relative to the projection system in the XY plane though there may be
some relative movement in the Z direction (in the direction of the
optical axis). See, for example, U.S. patent application Ser. No.
10/844,575, hereby incorporated in its entirety by reference. A seal is
formed between the liquid confinement structure and the surface of the
substrate.

[0061] Referring to FIG. 5, reservoir 10 forms a contactless seal to the
substrate around the image field of the projection system so that liquid
is confined to fill a space between the substrate surface and the final
element of the projection system. The reservoir is formed by a liquid
confinement structure 12 positioned below and surrounding the final
element of the projection system PL. Liquid is brought into the space
below the projection system and within the liquid confinement structure
12. The liquid confinement structure 12 extends a little above the final
element of the projection system and the liquid level rises above the
final element so that a buffer of liquid is provided. The liquid
confinement structure 12 has an inner periphery that at the upper end, in
an embodiment, closely conforms to the shape of the projection system or
the final element thereof and may, e.g., be round. At the bottom, the
inner periphery closely conforms to the shape of the image field, e.g.,
rectangular though this need not be the case.

[0062] The liquid is confined in the reservoir by a gas seal 16 between
the bottom of the liquid confinement structure 12 and the surface of the
substrate W. The gas seal is formed by gas, e.g. air or synthetic air
but, in an embodiment, N2 or another inert gas, provided under
pressure via inlet 15 to the gap between liquid confinement structure 12
and substrate and extracted via first outlet 14. The overpressure on the
gas inlet 15, vacuum level on the first outlet 14 and geometry of the gap
are arranged so that there is a high-velocity gas flow inwards that
confines the liquid. Such a system is disclosed in U.S. patent
application Ser. No. 10/705,783, hereby incorporated in its entirety by
reference.

[0063] In an embodiment, to facilitate imaging of the substrate, the
leveling and alignment of the substrate may be performed at an exposure
position of the substrate. In other words, a substrate level sensor (used
to facilitate focus of a patterned projection beam on the substrate) and
a substrate alignment sensor (used to facilitate proper lateral
positioning of the substrate relative to the patterned projection beam)
are provided around the projection system and/or the substrate positioned
adjacent the projection system, so that the substrate can be measured
when the substrate moves relative to and near the projection system
during exposure. In an immersion lithography apparatus, with the
structure used to provide or maintain liquid between the projection
system and the substrate, the amount of physical space remaining to
permit the provision or operation of one or both of those sensors is very
limited. Such space may be even more at a premium with larger projection
systems such as those having high numerical apertures (NA), such as about
1.3. Therefore, according to an embodiment, where the level sensor and/or
the alignment sensor use a measuring radiation beam, the measuring
radiation beam may wholly or partly pass through the projection system.

[0064]FIG. 6 schematically depicts passage of a patterned projection beam
and a measuring beam through an optical element of the projection system
of an immersion lithographic apparatus according to an embodiment of the
invention. A portion of an example projection system PL is shown. Liquid
11 is disposed between the projection system PL and the substrate W. A
patterned projection beam 20 is shown as entering the portion of the
projection system PL at two points (although as will be apparent, these
are just 2 rays representative of a wave). The patterned projection beam
20 passes through the portion of the projection system PL, then through
the liquid 11, and is focused onto the substrate W.

[0065] In this example, an incoming level sensor measuring beam 22 (e.g.,
provided by one or more laser sources, light emitting diodes, (halogen)
lamps, etc.) is shown entering the portion of the projection system PL.
The measuring beam passes through the portion of the projection system PL
and then through the liquid 11 onto the substrate. The measuring beam
reflects off the substrate W and then passes, as an outgoing level sensor
measuring beam 24, through the liquid 11 and the portion of the
projection system PL out to a level sensor detector (not shown). While a
level sensor measuring beam is shown and described in FIG. 6, the
measuring beam may instead or additionally be an alignment sensor
measuring beam or any other measuring beam.

[0066] While the patterned projection beam 20 and the level sensor
measuring beam 22,24 are shown as focused at substantially the same point
on the substrate to facilitate accurate leveling/focus measurements, the
beams need not be focused at the substantially same point. For example,
the level sensor measuring beam 22,24 may be focused at a position in
advance of where the patterned projection beam 20 will be focused so that
leveling/focusing calculations and adjustments can be made in advance of
the patterned beam projection beam 20 impinging the substrate W. Where
the measuring beam 22, 24 is, for example, an alignment beam, the
measuring beam 22, 24 may be focused at a different position, for
example, at an alignment mark, than the patterned projection beam 20.

[0067] Since a measuring beam should not expose the radiation sensitive
material of the substrate W, the wavelength of the radiation used for the
measuring beam is selected not to expose the radiation sensitive material
and thus is typically different than the wavelength of the radiation of
the patterned projection beam. For example, in the case of a level sensor
measuring beam, for rough capturing, HeNe laser radiation may be used for
the measuring beam, but to reduce thin film effects, broad band radiation
should be used instead or in addition.

[0068] However, in the case of level sensor, for example, using a
different wavelength for the measuring beam than for the patterned
projection beam, there will likely be a difference in the focus detected
by the level sensor passing its measuring beam through the projection
system from an actual focus associated with the patterned projection beam
passing through the same projection system. This is because one or more
projection system characteristics (such as refractive index) vary with
wavelength. Thus, a change of refractive index somewhere in the optical
path of the measuring beam (having a certain wavelength) may cause a
detected focus to differ from an actual focus associated with the
patterned projection beam (having a different wavelength). In addition or
alternatively, other measurement results using a measuring beam, such as
alignment, could similarly be affected. For example, in the case of an
alignment beam, the actual lateral position of the patterned projection
beam on the substrate may be different from the expected lateral position
of the patterned beam as determined by an alignment measurement using an
alignment measuring beam projected on an alignment mark on the substrate.

[0069] A change of the refractive index in the optical path of the
measuring beam may occur in any number of ways. Examples include:
[0070] temperature change of the liquid and/or the optical element
through which the measuring beam passes; [0071] pressure change of the
liquid and/or the optical element through which the measuring beam
passes; [0072] composition change of the liquid (e.g., contamination);
and [0073] pressure and/or temperature change in a flushing gas used to
condition the measuring beam path into and/or out of the projection
system.

[0074] Furthermore, a difference between a value measured using a
measuring beam and an actual value associated with a patterned projection
beam may also result from one or more other changes. For example, a
change in the wavelength of the patterned projection beam may cause a
measurement made using a measuring beam to be inaccurate. Similarly, a
change in the wavelength of the measuring beam may cause a measurement
made using that measuring beam to be inaccurate. Furthermore, movement
(manipulation) of one or more optical elements in the projection system
or substrate height movement may cause a difference between a value
measured using a measuring beam and an actual value associated with a
patterned projection beam.

[0075] Accordingly, in an embodiment, a metrology model / method is
implemented to correct for the difference between an exposure parameter
value measured using a measuring beam and an applied exposure parameter
value associated with a patterned projection beam attributable to the
difference in wavelength of the measuring beam and the projection beam.

[0076]FIG. 7 depicts a schematic flow chart of the metrology method
according to an embodiment of the invention.

[0077] At step 30, one or more sensors 26 measure one or more of the
physical property changes as described above at or near the time the
measuring beam measures an exposure parameter P (such as focus, substrate
height, and/or alignment). For example, a pressure sensor may measure the
pressure of the liquid, flushing gas and/or optical element through which
the measuring beam passes. Additionally or alternatively, a temperature
sensor may measure the temperature of the liquid, flushing gas and/or
optical element through which the measuring beam passes. In an
embodiment, multiple different types of measurements may be made (e.g.,
pressure and temperature measurement) and/or multiple measurements of the
same type may be made (e.g., multiple pressure measurements). The number
of measurements are designated by the symbol j. The one or more measured
physical property values may then be denominated as array X_j. In an
embodiment, one or more sensors provide a measured physical property
value as a difference with respect to a nominal value, e.g., at which the
lithographic apparatus was optimally adjusted. For example, a temperature
sensor may provide the difference between the actual temperature measured
(e.g., 22.3° C.) and a nominal temperature at which the
lithographic apparatus was optimally adjusted (e.g., 22.1° C.) as
the temperature measurement (i.e., 0.2° C.). In an embodiment, it
will be appreciated that the method may be extended to situations where a
nominal value (i.e., calibration) has not been established through the
use of additional calculations and/or measurements.

[0078] At step 32, the impact of the one or more measured physical
property values X_j on the exposure parameter P is determined or
obtained. This impact may be designated as the derivative dP/dX_j. For
example, where the exposure parameter P is focus and the measured
physical property values X_j are temperature and pressure of the
immersion liquid, dP/dX_j may represent the rate of change of focus with
respect to temperature and pressure.

[0079] In an embodiment, dP/dX_j may be determined at, before or after
step 32. In other words, dP/dX_j may be determined off-line, i.e., before
the measuring beam measures the exposure parameter P, or on-line, i.e.,
at or near the time the measuring beam measures the exposure parameter P.
In an embodiment, dP/dX_j may be determined by experiment/calibration,
from empirical results, and/or by optical theory (e.g., optical ray
tracing). For example, dP/dX_j may be calculated from the application of
materials values (such as a table of values for a particular physical
property at different conditions, e.g., the refractive index of a
material at various temperatures) to relevant physics and/or mathematical
formulas. In another example, dP/dX_j may be determined by experimental
measurement. Furthermore, dP/dX_j may be determined for multiple exposure
and/or measuring beam wavelengths and may be a difference between the
value for the exposure beam wavelength and the measuring beam wavelength.
Additionally or alternatively, dP/dX_j may be determined per projection
system type (e.g., a projection system used in multiple lithographic
apparatus) or individually per specific projection system in a
lithographic apparatus.

[0080] At step 34, the measured exposure parameter P is corrected at least
in part to take account of the measured physical property values X_j. In
an embodiment, this correction may be formulated as:

P_applied=P_measured-sum (j) {X--j*dP/dX--j}

where P_applied is the exposure parameter to be applied, for example,
during exposure of the substrate by a patterned projection beam and
P_measured is the exposure parameter as measured using the measuring
beam. Thus, the term "sum (j) (X_j*dP/dX_j)" is the cumulative offset to
be applied to the measured exposure parameter to obtain an exposure
parameter to be applied, for example, during exposure of the substrate.
The cumulative offset is the sum of the respective offsets attributable
to each of the measured physical values X_j. Matrix operations may be
used for the correction so as to address possible cross terms between
certain physical properties, such as projection system manipulator and
environmental (e.g., temperature, pressure) dependencies. The
determination and/or application of the exposure parameter to be applied
may be performed continuously or at intermittent times.

[0081] So, in an example, a level sensor may project a measuring beam
through an optical element of the projection system of a lithographic
apparatus and through an immersion liquid to measure a height of the
substrate F measured during exposure of the substrate, the measured
height corresponding to a focus for the patterned projection beam. This
measured height (and thus focus) may then be corrected to take account of
the temperature (X_1) and pressure (X_2) of the immersion liquid to yield
a corrected height F applied (and thus focus) to be applied during
exposure of the substrate. A corrective function of the focus with
respect to the temperature (dF/dX_1) and pressure (dF/dX_2) is provided
(e.g., by experiment or empirical results). The corrective function is
then multiplied with the measured values of the pressure and temperature
and added together (sum(j) {X_j*dF/dX_j}) to yield a cumulative offset to
be applied to the measured height to yield the corrected height (and thus
corrected focus). This may be summarized by the formula
F_applied=F_measured-sum(j) {X_j*dF/dX_j}.

[0082] In another example, an alignment sensor may project a measuring
beam through an optical element of the projection system of a
lithographic apparatus and through an immersion liquid to measure a X-Y
position of the substrate LP measured during exposure of the substrate,
the measured X-Y position corresponding to a lateral placement of the
patterned projection beam. This measured X-Y position (and thus lateral
placement) may then be corrected to take account of the temperature (X_1)
and pressure (X_2) of the immersion liquid to yield a corrected X-Y
position LP_applied (and thus lateral placement) to be applied during
exposure of the substrate. A corrective function of the lateral placement
with respect to the temperature (dLP/dX_1) and pressure (dLP/dX_2) is
provided (e.g., by experiment or empirical results). The corrective
function is then multiplied with the measured values of the pressure and
temperature and added together (sum(j) {X_j*dLP/dX_j}) to yield a
cumulative offset to be applied to the measured X-Y position to yield the
corrected X-Y position (and thus corrected lateral placement). This may
be summarized by the formula LP_applied=LP_measured-sum(j)
{X_j*dLP/dX_j}.

[0083] In an embodiment, the measured exposure parameter may be
specifically corrected for the wavelength of the measuring beam and of
the exposure beam to yield the exposure parameter to be applied. For
example, this may be formulated as:

where P_applied is the exposure parameter, such as focus or lateral
placement, to be applied, for example, during exposure of the substrate
by a patterned projection beam, P_measured is the exposure parameter as
measured using the measuring beam, Δ(MV) is the difference in the
measured physical property value, such as temperature or pressure, from a
nominal value (typically a value at which the lithographic apparatus was
optimally configured), dP/dMV(measuring beam wavelength) is the rate of
change of the exposure parameter with respect to the measured physical
property value for the measuring beam wavelength, and dP/dMV(exposure
wavelength) is the rate of change of the exposure parameter with respect
to the measured physical property value for the measuring beam
wavelength.

[0084] A lithographic apparatus may have various constants defined to
provide good or optimal performance. For example, the lithographic
apparatus may have one or more constants associated with the projection
system such as positioning of one or more optical elements in the
projection system and/or one or more constants associated with the
alignment system such as alignment mark configuration and/or location. In
embodiment, the metrology model may be implemented to correct for the
wavelength dependency of these constants. In other words, one or measured
physical property values, such as pressure and/or temperature, may be
used in association with, for example, a rate of change of the constant
relative to the measured value or a rate of change of a relevant exposure
parameter relative to the measured value (e.g. dalignment/dtemperature or
dfocus/dpressure) to yield a corrected constant.

[0085] To implement the method described above, a correction system may be
provided to a lithographic apparatus which is configured or programmed to
perform an embodiment of the method as described herein. The correction
system may be a computer program incorporated into a processor or sensor
of the lithographic apparatus. Further, a computer program product (e.g.,
a software program on a disk or in a memory) may be provided to perform
an embodiment of the method as described herein.

[0086] In an embodiment, the last optical element of the projection system
is stiff mounted to the rigid body of the remainder of the projection
system (e.g., lens body). Due to static forces (immersion liquid height
and/or over pressure difference of the projection system) and dynamic
forces (immersion liquid flow and/or dynamic environmental pressure
change), the last optical element may change in height. In an immersion
lithography system, this may lead to a change of the optical path which,
between alignment measurements, may lead to mainly focus and/or spherical
aberration drift. For a projection system NA of 0.75, 193 nm radiation
and water as the immersion liquid, the sensitivity may be ≠1 nm
defocus and 9 pm Z9 drift per nm Z-displacement. With a stiffness of
5×106 and a maximum allowable Z9 drift of 0.5 nm, this could
lead to a maximum force of 0.3 N on the optical element connected to the
liquid--a force that may occur in an immersion lithographic apparatus.

[0087] Thus, in an embodiment, a sensor may be provided to measure a
height of an optical element configured to be connected to the liquid
(e.g., the last optical element of the projection system). In an
embodiment, the sensor may be implemented as part of an existing level
sensor provided to measure a height (e.g. with respect to the last
optical element of the projection system) of the substrate or of another
object such as the substrate table, including any sensor, fiducial, etc.
In an embodiment, the sensor may be a separate sensor, from the level
sensor, for this purpose.

[0088] In an embodiment, the sensor to measure a height of the optical
element is an optical sensor configured to detect a measuring beam
traveling from the optical element to measure the height of the optical
element and optionally to project the measuring beam at the optical
element. Referring to FIG. 8, an arrangement for the optical sensor 46 is
schematically depicted. An incoming measuring beam 40 (e.g, provided by
one or more laser sources, light emitting diodes, (halogen) lamps, etc.)
is shown directed at the optical element OE, which optical element is
connected to the liquid 11. The measuring beam reflects off a top surface
of the optical element OE, as an outgoing measuring beam 42, to the
sensor detector 46, which is used to determine the height of the optical
element. In an embodiment, the measuring beam is a level sensor measuring
beam although it may instead or additionally be an alignment sensor
measuring beam or any other measuring beam. In an embodiment, the
measuring beam may be reflected off a bottom surface of the optical
element OE (e.g., the surface that is the interface between the optical
element OE and the liquid 11). When reflected off the bottom surface, the
beam may pass through the top surface of the optical element OE to the
bottom surface, where the beam is reflected toward and out the top
surface. Alternatively, the beam may be projected directly onto the
bottom surface by, e.g., a beam emanating or reflected from below the
optical element OE.

[0089] In an embodiment, the sensor may be configured as part of a level
sensor 46 used to measure the height of the substrate (or other object).
An incoming level sensor measuring beam 40 (e.g., provided by one or more
laser sources, light emitting diodes, (halogen) lamps, etc.) is shown
entering the optical element OE. The measuring beam passes through the
optical element OE and then through the liquid 11 onto the substrate (or
other object). The measuring beam reflects off the substrate (or other
object) and then passes, as an outgoing level sensor measuring beam 44,
through the liquid 11 and the optical element OE out to a level sensor
detector 46. A portion of the incoming level sensor measuring beam 40
reflects off a top surface of the optical element OE, as an outgoing
measuring beam 44, to the sensor detector 46. Alternatively, the incoming
measuring beam used to create the outgoing measuring beam 44 could be
directed at a different angle or have a different wavelength in order to
cause it to reflect off the optical element OE. The outgoing level sensor
measuring beam 42 and the outgoing measuring beam 44 created an
interference area. By analyzing the fringes of the interference area, the
difference in height in between the substrate (or the object) and the
optical element can be detected or determined and thus the change in
height of the optical element can be determined.

[0090] In an alternative or additional embodiment, the sensor may be a
mechanical, ultrasonic, magnetic and/or electrical sensor to measure the
height or change in height of the optical element. Referring to FIG. 9,
sensor 46 is a mechanical, ultrasonic, magnetic and/or electrical sensor
used to measure a height of the optical element OE.

[0091] In each case, the measured height from the sensor may be used to at
least partly correct for a change in the measured height of the optical
element. In an embodiment, a signal may be sent from the sensor 46 to a
servo system 50 which controls a positioning system PW of the substrate
table WT so that the height of substrate (or other object) can be
adjusted to at least partly correct for the change in measured height of
the optical element. Additionally or alternatively, a signal may be sent
to a servo system 50 used to control the position of the optical element
height so that the height of the optical element can be adjusted to at
least partly correct for the change in measured height of the optical
element.

[0092] Through this mechanism, focus and spherical aberration caused by
changing forces on the optical element of the projection system connected
to the liquid may be stabilized and/or reduced.

[0093] For further clarity, the term height includes change in height and
may include tilt. Additionally, while the concepts herein have been
described in the context of a lithography apparatus, they might equally
be applied to other apparatus that use a liquid between an optical
element and surface of an object. For example, the concepts herein may be
applied to an immersion metrology apparatus that uses a beam of radiation
projected through a liquid to measure characteristics of an object.

[0094] In European Patent Application No. 03257072.3, the idea of a twin
or dual stage immersion lithography apparatus is disclosed. Such an
apparatus is provided with two tables for supporting a substrate.
Leveling measurements are carried out with a table at a first position,
without immersion liquid, and exposure is carried out with a table at a
second position, where immersion liquid is present. Alternatively, the
apparatus has only one table. In a preferred embodiment, the apparatus,
method and/or computer program product as described herein is applied to
a single stage/table lithography apparatus.

[0095] Although specific reference may be made in this text to the use of
lithographic apparatus in the manufacture of ICs, it should be understood
that the lithographic apparatus described herein may have other
applications, such as the manufacture of integrated optical systems,
guidance and detection patterns for magnetic domain memories, flat-panel
displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.
The skilled artisan will appreciate that, in the context of such
alternative applications, any use of the terms "wafer" or "die" herein
may be considered as synonymous with the more general terms "substrate"
or "target portion", respectively. The substrate referred to herein may
be processed, before or after exposure, in for example a track (a tool
that typically applies a layer of resist to a substrate and develops the
exposed resist), a metrology tool and/or an inspection tool. Where
applicable, the disclosure herein may be applied to such and other
substrate processing tools. Further, the substrate may be processed more
than once, for example in order to create a multi-layer IC, so that the
term substrate used herein may also refer to a substrate that already
contains multiple processed layers.

[0096] The terms "radiation" and "beam" used herein encompass all types of
electromagnetic radiation, including ultraviolet (UV) radiation (e.g.
having a wavelength of or about 365, 248, 193, 157 or 126 nm).

[0097] The term "lens", where the context allows, may refer to any one or
combination of various types of optical components, including refractive
and reflective optical components.

[0098] While specific embodiments of the invention have been described
above, it will be appreciated that the invention may be practiced
otherwise than as described. For example, where applicable, an embodiment
of the invention may take the form of a computer program containing one
or more sequences of machine-readable instructions describing a method as
disclosed above, or a data storage medium (e.g. semiconductor memory,
magnetic or optical disk) having such a computer program stored therein.
For example, the metrology model/method may be implemented as a computer
program and the computer program may interact with the lithographic
apparatus to obtain measured data (e.g., obtain a measured focus from one
or more level sensors of the lithographic apparatus and/or obtain a
measured lateral placement from one or more alignment sensors of the
lithographic apparatus) and return corrected data (e.g., return the
measured focus as corrected to account for the different wavelength of
the level sensor(s) measuring beam and/or return the measured lateral
placement as corrected to account for the different wavelength of the
alignment sensor(s) measuring beam).

[0099] One or more embodiments of the invention may be applied to any
immersion lithography apparatus, in particular, but not exclusively,
those types mentioned above and whether the immersion liquid is provided
in the form of a bath or only on a localized surface area of the
substrate. A liquid supply system as contemplated herein should be
broadly construed. In certain embodiments, it may be a mechanism or
combination of structures that provides a liquid to a space between the
projection system and the substrate and/or substrate table. It may
comprise a combination of one or more structures, one or more liquid
inlets, one or more gas inlets, one or more gas outlets, and/or one or
more liquid outlets that provide liquid to the space. In an embodiment, a
surface of the space may be a portion of the substrate and/or substrate
table, or a surface of the space may completely cover a surface of the
substrate and/or substrate table, or the space may envelop the substrate
and/or substrate table. The liquid supply system may optionally further
include one or more elements to control the position, quantity, quality,
shape, flow rate or any other features of the liquid.

[0100] The immersion liquid used in the apparatus may have different
compositions, according to the desired properties and the wavelength of
exposure radiation used. For an exposure wavelength of 193 nm, ultra pure
water or water-based compositions may be used and for this reason the
immersion liquid is sometimes referred to as water and water-related
terms such as hydrophilic, hydrophobic, humidity, etc. may be used.

[0101] More generally, each step of the method may be executed on any
general computer, such as a mainframe computer, personal computer or the
like and pursuant to one or more, or a part of one or more, program
modules or objects generated from any programming language, such as C++,
Java, Fortran or the like. And still further, each step, or a file or
object or the like implementing each step, may be executed by special
purpose hardware or a circuit module designed for that purpose. For
example, the invention may be implemented as a firmware program loaded
into non-volatile storage or a software program loaded from or into a
data storage medium as machine-readable code, such code being
instructions executable by an array of logic elements such as a
microprocessor or other digital signal processing unit.

[0102] The invention may be implemented as an article of manufacture
comprising a computer usable medium having computer readable program code
means therein for executing the method steps of the invention, a program
storage device readable by a machine, tangibly embodying a program of
instructions executable by a machine to perform the method steps of the
invention, a computer program product, or an article of manufacture
comprising a computer usable medium having computer readable program code
means therein, the computer readable program code means in said computer
program product comprising computer readable code means for causing a
computer to execute the steps of the invention. Such an article of
manufacture, program storage device, or computer program product may
include, but is not limited to, CD-ROMs, diskettes, tapes, hard drives,
computer system memory (e.g. RAM or ROM) and/or the electronic, magnetic,
optical, biological or other similar embodiment of the program
(including, but not limited to, a carrier wave modulated, or otherwise
manipulated, to convey instructions that can be read, demodulated/decoded
and executed by a computer). Indeed, the article of manufacture, program
storage device or computer program product may include any solid or fluid
transmission medium, magnetic or optical, or the like, for storing or
transmitting signals readable by a machine for controlling the operation
of a general or special purpose computer according to the method of the
invention and/or to structure its components in accordance with a system
of the invention.

[0103] The invention may also be implemented in a system. A system may
comprise a computer that includes a processor and a memory device and
optionally, a storage device, an output device such as a video display
and/or an input device such as a keyboard or computer mouse. Moreover, a
system may comprise an interconnected network of computers. Computers may
equally be in stand-alone form (such as the traditional desktop personal
computer) or integrated into another apparatus (such as a lithographic
apparatus).

[0104] The system may be specially constructed for the required purposes
to perform, for example, the method steps of the invention or it may
comprise one or more general purpose computers as selectively activated
or reconfigured by a computer program in accordance with the teachings
herein stored in the computer(s). The system could also be implemented in
whole or in part as a hard-wired circuit or as a circuit configuration
fabricated into an application-specific integrated circuit. The invention
presented herein is not inherently related to a particular computer
system or other apparatus. The required structure for a variety of these
systems will appear from the description given.

[0105] In the case of diagrams depicted herein, they are provided by way
of example. There may be variations to these diagrams or the steps (or
operations) described herein without departing from the spirit of the
invention. For instance, in certain cases, the steps may be performed in
differing order, or steps may be added, deleted or modified. All of these
variations are considered to comprise part of the invention as recited in
the appended claims.

[0106] The descriptions above are intended to be illustrative, not
limiting. Thus, it will be apparent to one skilled in the art that
modifications may be made to the invention as described without departing
from the scope of the claims set out below.